The present disclosure relates to an alignment system and method for positioning and/or keeping a first object at a controlled distanced with respect to a second object.
In nano-manufacturing and metrology, it is often desired to have an optical element accurately positioned with respect to a substrate, e.g. wafer. Typically, a measurement is done initially with a level sensor, which maps the height of a target substrate, followed by the optical element being positioned based on the level sensing measurement. For example, in a lithographic apparatus an imaging lens can be used to project an image onto a wafer. To maintain focus, it is desired to keep a controlled distance between the imaging lens and the wafer. However, at high resolutions, even minute variations in the thickness, tilting, or waviness of the wafer can have a significant influence on the focus, e.g. while moving the wafer under the lens. Accordingly, it is desired to constantly monitor and align the distance at high precision.
For example, the distance can be measured using optical means such as interferometry. However, optical techniques are typically substrate dependent, e.g. affected by the reflection behavior of the substrate material. Alternatively, capacitive sensors can be used, but they are also substrate dependent, e.g. affected by the conductivity of the substrate. Alternatively, U.S. Pat. No. 7,021,120 discloses a high resolution gas gauge proximity sensor which may be less dependent on the substrate. However, the system comprises a complicated gas system and may affect vacuum conditions. Also the resolution may still be insufficient and the system relatively slow.
For example, WO 2014/188379 A1 describes an evaluation system that comprises a solid immersion lens and multiple atomic force microscopes (AFMs) arranged to generate information indicative of a spatial relationship between the solid immersion lens and a substrate. The system comprises a location correction element and a controller arranged to receive the spatial relationship information and to send correction signals to the location correction element for introducing a desired spatial relationship between the solid immersion lens and the substrate. A supporting structure is coupled to the spatial sensors, the solid immersion lens and the at least one location correction element. Unfortunately, the relative distance between the lens and substrate cannot be accurately controlled to adapt for different circumstances.
For example, U.S. Pat. No. 5,508,527 describes a method and apparatus for detecting the relative positional displacement between a mask and a wafer. The mask is provided with a cantilever. The position mark is provided on the wafer so as to face the cantilever. A relative positional displacement between the mask and the wafer is detected from a deformation amount of the cantilever based on a force acting between the position mark and the cantilever upon relative movement of the position mark and the cantilever. Unfortunately, the relative distance between the mask and wafer cannot be easily controlled to adapt for different circumstances.
Accordingly, it is desired to provide a substrate independent alignment system to easily set and accurately maintain distance between objects at high precision.